Immunity Against Fungi

REVIEW Immunity against fungi Michail S. Lionakis,1 Iliyan D. Iliev,2 and Tobias M. Hohl3 1Fungal Pathogenesis Unit, Laboratory of Clinical Infectious Diseases, National Institute of Allergy and Infectious Diseases, NIH, Bethesda, Maryland, USA. 2Jill Roberts Institute for Research in IBD, Department of Medicine, Weill Cornell Medical College, New York, New York, USA. 3Infectious Disease Service, Department of Medicine, and Immunology Program, Memorial Sloan Kettering Cancer Center, New York, New York, USA. Pathogenic fungi cause a wide range of syndromes in immune-competent and immune- compromised individuals, with life-threatening disease primarily seen in humans with HIV/AIDS and in patients receiving immunosuppressive therapies for cancer, autoimmunity, and end-organ failure. The discovery that specific primary immune deficiencies manifest with fungal infections and the development of animal models of mucosal and invasive mycoses have facilitated insight into fungus-specific recognition, signaling, effector pathways, and adaptive immune responses. Progress in deciphering the molecular and cellular basis of immunity against fungi is guiding preclinical studies into vaccine and immune reconstitution strategies for vulnerable patient groups. Furthermore, recent work has begun to address the role of endogenous fungal communities in human health and disease. In this review, we summarize a contemporary understanding of protective immunity against fungi. Introduction Humans are exposed to fungi throughout life via inhalation, digestion, and/or traumatic inoculation of fungal particles. The vast majority of these encounters are asymptomatic, and less than 100 of the estimat- ed 5 million fungi species are associated with human disease (1) (Table 1 and Figure 1). Fungi can either exist as spherical yeast cells (e.g., Cryptoccocus neoformans) or as molds that form branching tubular hyphae (e.g., Aspergillus fumigatus). Dimorphic fungi (e.g., Histoplasma capsulatum) grow as molds in the environ- ment and yeasts in human tissue. Candida albicans grows as yeast cells and pseudohyphae, a hyphal form with tapered ends, in human tissue; this morphologic switch is essential for virulence (2). Fungi were recognized to cause disease during investigations into the scalp dermatophyte infection favus, which was widespread in 19th century Europe (3). German physiologist Robert Remak (1815–1865) immersed favus skin samples in acetic acid and observed fungal hyphae and conidia (named Trichophyton schönleinii in honor of Johann Schönlein, Remak’s mentor). In 1842, Remak injected favus crust–isolated material into his forearm and noted growth in the lesions, thereby establishing causality between the fungus and disease. Several events, including the advent of myeloablative chemotherapy for neoplasia, glucocorticoids and immune modulators for autoimmunity, transplantation for end-organ failure, and the AIDS pan- demic, contributed to the emergence of fungal infections in the second half of the 20th century. Novel pathogenic fungi that pose a threat to humans (e.g., Cryptococcus gattii), amphibians (e.g., Batrachochytri- um dendrobatidis), and bats (e.g., Pseudogymnoascus destructans) have also been identified (4). In response, research in fungal pathogenesis and antifungal immunity has intensified to inform vaccine- and thera- py-based approaches for mycoses (5). This review focuses on insights gained from animal models and patients with primary immune deficiency disorders (PIDDs), but does not cover allergenic or toxin- mediated fungal disease (6, 7). Conflict of interest: The authors have Antifungal immunity from the bench: contribution of animal models declared that no conflict of interest This section focuses on antifungal immunity to different yeasts, molds, and dimorphic fungi. Contempo- exists. rary animal models of fungal infection are reviewed elsewhere (8). Published: June 2, 2017 Fungal recognition and immune activation. The fungal cell wall contains polysaccharide and lipid moi- Reference information: eties that activate immune responses (9) (Table 2). The cell wall is exterior to the plasma membrane and JCI Insight. 2017;2(11):e93156. https:// arranged in layers: the innermost layer typically consists of chitin, an N-acetylglucosamine polymer; the doi.org/10.1172/jci.insight.93156. adjacent external layer is formed by immunoreactive β-(1,3) and β-(1,6) glucans, which are concealed by insight.jci.org https://doi.org/10.1172/jci.insight.93156 1 REVIEW many fungi. H. capsulatum employs an α-glucan layer and the action of a glucanase (9–11). A. fumigatus resting conidia utilize a proteinaceous hydrophobin layer (12), while the hyphal cell wall layer contains galactomannan and galactosaminogalactan, the latter of which conceals inflammatoryβ -glucan (13, 14). The C. albicans outer cell wall consists of glycoproteins that incorporate N- and O-linked mannans and induces inflammatory responses via the mannose receptor and TLR-4 (15).C . albicans mannans conceal β-glucans as well; the latter are exposed on bud and birth scars during yeast cell division (9). The C. neoformans capsule covers the chitinous and β-glucan–rich cell wall layers and largely consists of glucuronoxylomannan and galactoxylomannan (16). Previously published reviews provide in-depth discussion of fungal cell wall architecture (9, 16–18). At portals of entry fungal cells encounter and bind to antibodies, complement, and soluble pat- tern recognition receptors. Collectively, these interactions facilitate signaling responses by membrane- bound receptors and the induction of antifungal effector mechanisms (5, 9). In the lung, the collectin pentraxin-3 (PTX3) binds to A. fumigatus conidial galactomannan (19) and facilitates complement deposition and CD32-dependent conidial uptake by neutrophils (20). Ptx3–/– mice are susceptible to respiratory A. fumigatus challenge (19), and a PTX3 polymorphism enhances the vulnerability of hema- topoietic cell transplant recipients to invasive aspergillosis (21). The C-type lectin receptor (CLR) dectin-1 (encoded by Clec7a) binds β-glucans from a variety of fungi, including those on C. albicans bud scars and germinating A. fumigatus conidia, and activates signaling responses to Pneumocystis jiroveci, H. capsulatum, Coccidioides posadasii, and Paracoccidioides brasil- iensis (5, 9, 22–24) (Figure 2). β-Glucan binding displaces regulatory phosphatases CD45 and CD148 (25), induces SRC-dependent phosphorylation of the intracellular ITAM-like motif, and recruits the SHP-2 phosphatase (26). SYK docks to this scaffold and transduces signals via PKC-δ (27) and the VAV family of GEFs (28) to CARD9, which complexes with BCL10 and MALT1 (29) to activate the canonical NF-κB subunits p65 and c-REL (30). Dectin-1 signaling also modulates the noncanonical NF-κB subunit RELB through RAF-1–dependent phosphorylation and deacetylation (31). In macrophages and dendritic cells (DCs), the CARD9/BCL10/MALT1 complex directs Il1b transcription and caspase-1– and caspase-8–dependent IL-1β release (32, 33), in part via the activity of NRLP3- and AIM2-containing inflammasomes (34). Rubicon can disrupt signal transduction and NF-κB activation via the CARD9/BCL10/MALT1 complex (35). Dectin-1/SYK/CARD9–dependent cytokines, such as TNF, CXCL2, IL-6, IL-23, and IL-1β, promote innate immune activation and Th17 differentiation (36). In addition, dectin-1/SYK signaling in DCs induces IFN-β production via IRF5 (37). The role of type I IFN signaling in defense against candidiasis remains controversial, with both protective (37) and detrimental (38) phenotypes reported. Dectin-1 signaling regulates ERK (also known as MAP kinase) activity via H-RAS and RAS guanine nucleotide–releasing factor 1 (RASGRF1) (39). This pathway regulates macrophage IL-6, IL-1β, and TNF, but not IL-12 responses, and is protective during systemic candidiasis (39). c-JUN kinase isoform 1–deficient (JNK1-deficient) mice are resistant to systemic candidiasis (40). Dectin-1–induced JNK1 signaling negatively regulates CD23 (encoded by Fcer2) expression via nuclear factor of activated T cells (NFAT) activation. CD23 binds α-mannans and β-glucans and induces the antifungal effector NOS2. Consistent with this model, Cd23–/– mice are susceptible to systemic candidiasis (40). In otherwise nonphagocytic cells, dectin-1 expression promotes phagocytosis of nonopsonized β-glucan particles (5, 9). Bruton’s tyrosine kinase (BTK) and VAV-1 interact with dectin-1 in macrophages during C. albicans phagocytosis, a process impaired by genetic loss of either protein (41). Dectin-1/SYK/ CARD9 signaling in NADPH oxidase activity is controversial, as dectin-1–dependent (24) and –indepen- dent (42) control of β2 integrin (CD18) activation and the respiratory burst have been reported in vitro. Murine Clec7a –/– and Card9–/– neutrophils display no cell-intrinsic defect in killing A. fumigatus conidia, unlike p47phox–/– neutrophils (43). These data can be reconciled if the major role of dectin-1/CARD9 is to modulate NADPH oxidase and fungal killing via soluble mediators, rather than by cell-intrinsic activation. β2 integrins and TLR signaling can collaborate with dectin-1 to mount macrophage inflammatory responses to H. capsulatum and other fungi (44, 45). Dectin-2 (encoded by Clec4n) forms a complex with dectin-3 (encoded by Clec4d) to bind Candida α-mannans (46, 47) or with mincle (encoded by Clec4e) to bind Malassezia glycolipids (48). Blastomyces der- matiditis, H. capsulatum, C. posadasii, and A. fumigatus induce dectin-2

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